PURPOSE: To obtain good operation of a blast furnace by detecting molten zone position by gas analyzed values sampled from plural gas sampling tubes at different heights of the furnace to continuously detect this position.

CONSTITUTION: Plural pieces of the gas sampling tubes are arranged at multiple steps toward height direction of the blast furnace. The gas is sampled from these gas sampling tubes, respectively, to measure gas utilization ratio shown by CO2/(CO+CO2). Next, the position, where the gas utilization ratio becomes zero, is detected, for example by curve-fitting from relation these measured values and these measured position, or directly by measuring. Then, this position is presumed to be the molten zone position in the blast furnace.